Fuel cells are primary generators of electrical power. Fuel cells are similar to chemical batteries, such as lead or alkaline batteries, in that electricity is generated from the reaction of the fuel and the oxidant. Unlike chemical batteries, however, the fuel and oxidant in a fuel cell is continuously resupplied. Thus, a fuel cell never has to be electrically recharged. The cell only requires a new supply of fuel and oxidant for continued operation.
A fuel cell consists of two chambers, one containing the fuel, usually hydrogen, and the second containing an oxidant, usually oxygen or an oxygen-rich gas such as air. The hydrogen and oxidant chambers sandwich two electrodes which in turn surround an electrolyte. Hydrogen molecules are adsorbed at one electrode (the anode) to break the hydrogen molecular bonds, creating hydrogen ions and free electrons. These electrons, as will be discussed, flow from the anode to a load device, such as a light bulb, and flow on to the other electrode, the cathode. Oxygen molecules are adsorbed at the cathode, and the hydrogen ions, migrating through the electrolyte, react with the oxygen molecules at the cathode in a reduction reaction to produce water. The adsorption of the hydrogen and oxygen molecules is stimulated by the use of a catalyst layer serving as the interface of each of the two electrodes and the electrolyte. The potential difference existing between the hydrogen and oxygen electrodes (anode and cathode, respectively) thus creates an electrical current. Once the electrons reach the cathode, they are consumed by the reduction reaction.
Fuel cells which use an ionomer membrane as the electrolyte have a significant advantage over fuel cells which use a liquid electrolyte system. Liquid electrolyte systems, such as the alkaline or phosphoric acid systems, require complex subsystems to assure the purity of the electrolyte, its continuous circulation and, most importantly, that a fixed three-phase boundary is maintained. The three-phase boundary is the interface at which the reactant gases, the catalyst and the electrolyte meet. Unless precise controls are maintained, the liquid electrolyte can flood the three-phase boundary and thereby prevent the reactant gases from efficiently reaching the catalyst.
An ionomer membrane, e.g., the DuPont product known as Nafion.RTM., eliminates the need for complex electrolyte subsystems and the precise controls otherwise necessary to maintain a fixed, three-phase boundary in a fuel cell. Nafion.RTM. is a proton exchange type of ionomer membrane. Acid groups, bonded within the membrane, facilitate the transit of protons from one side of the membrane to the other. Hydrogen ions are the typical species of proton which is transported using a proton exchange membrane. The transport of hydrogen ions within the membrane proceeds via a Grothius chain mechanism and, therefore, four to six water molecules are required for each hydrogen ion transported. If the ionomer membrane is not sufficiently hydrated, reduced hydrogen ion transfer will occur, and the fuel cell's performance will degrade. In extreme cases, dehydration of the membrane at elevated temperatures can lead to cracking of the membrane and loss of its ion-conducting capability. Thus, while ionomer membranes represent an advance over liquid electrolyte systems for fuel cell purposes, they present their own unique problems in designing a practical fuel cell assembly.
In order to deal with the dehydration problem, which is inherent to the use of an ionomer membrane, other fuel cell designers have chosen to use a hydrophobic electrode in conjunction with the membrane. A hydrophobic electrode will tend to retain water within the membrane and thereby reduce the overall loss of water from fuel cell assembly during operation. Hydrophobic fuel cell electrodes are typically composed of high surface area carbon particles, a graphite cloth backing layer and Teflon.RTM.. Teflon.RTM. particles, dispersed in an aqueous suspension, and the carbon particles are mixed. The mixture is applied to the graphite cloth backing layer. The electrode is then heated in order to sinter the Teflon.RTM. particles. The sintered Teflon.RTM. particles are hydrophobic and also serve to provide channels for the reactant gasses to reach the three-phase boundary. The electrical current produced at the catalyst layer flows via the carbon particles to the graphite cloth backing and then to a current collector. A typical hydrophilic electrode is the Prototech.RTM. electrode, manufactured by E-Tech in Ohio.
Unfortunately, hydrophobic electrodes present significant water management problems also. Indeed, water management has been a continuing and vexatious problem when hydrophobic electrodes are used in conjunction with a proton exchange membrane. In order to supply sufficient humidification for the membrane, water must be condensed onto the hydrogen-side electrode through the use of a separate complex subsystem.
Thus, state-of-the-art fuel cell electrodes, when used in conjunction with an ionomer membrane, have two principal deficiencies. First, the electrodes rely upon a pressure contact between carbon particles for electrical conductivity. Even with a graphite cloth backing, there is significant electronic resistance within the electrode because of this type of structure. Second, maintaining humidification of the membrane is difficult and requires an elaborate subsystem.